Organic isocyanates have widespread industrial uses. One method of manufacture of most commercial isocyanates involves the reaction of amine precursors with carbonyl chloride. During the process, carbamoyl chlorides are formed first which decompose to provide the desired isocyanates. Typically, for each mole of isocyanate group created, two molar equivalents of hydrogen chloride are generated. Once generated, hydrogen chloride may react with the functional groups in the amine precursors, the intermediates and the isocyanate products generating various byproducts. In the case of the preparation of alkoxysilyl group-containing isocyanates, hydrogen chloride, upon generation, reacts with the alkoxysilyl groups in the isocyanate products, generating chlorosilanes and the corresponding alcohols, which leads to the formation of further side products, thus lowering the overall yield of the target isocyanates.
In an effort to solve the problem, U.S. Pat. No. 4,654,428 discloses the use of tertiary amines such as triethyl or tripropyl amines in the reaction mixture. In the process, hydrogen chloride is neutralized by the tertiary amines forming amine hydrochloride salts, which precipitate and are subsequently filtered off. The isocyanate product is obtained by distillation of the filtrate.
However, there are a number of disadvantages associated with this process. Firstly, the amine salts formed from the '428 tertiary amines and hydrogen chloride are typically a high volume, low density, and fluffy solid, which is difficult and expensive to handle. Secondly, the distilled product made from the process disclosed in the '428 patent can be contaminated with the amine hydrochloride salts dissolved in the filtrate and/or hydrogen chloride formed by dissociation of the amine salts. To obviate the disadvantages, the '428 patent discloses treating the filtrate or the distillate with an alkali metal or alkaline earth metal salt of a carboxylic acid. This additional treatment step adds costs to the process. Thirdly, in practice, to ensure the efficient quenching of the hydrogen chloride, excess amine is used, which leads to an additional step of stripping off the excess amine from the final product, thus further adding costs to the process.
Accordingly, there is a continuing need in the isocyanato silane manufacturing industry for an enhanced process to produce the desired products without the additional steps and shortfalls of the prior art processes.